1. Field of the Invention
The invention concerns a hydraulic drive system with a consumer of hydraulic power connected to a pressure medium source and which is actuatable by means of a throttling control valve throttling in intermediate position, the opening width of which is determined by a power difference derived from a prescribed control signal and the load pressure of the consumer and is created at the control valve.
2. Description of the Art
Such a drive system is known from the DE-OS 31 46 561, where the control signal is formed by a control pressure. In contrast to the otherwise conventional drive system in which the opening width of the control valve is determined exclusively by the level of the control pressure and thus a definite volume stream of pressure medium and a definite movement speed of the consumer are prescribed, in a drive system of this type the pressure medium flows to the consumer with a quite specific pressure or a specific pressure difference on the consumer side, such that the consumer is moved with an accordingly prescribed force or a prescribed torque. Such a drive system can be used for both translatory and rotatory consumers. The drive system is designed so that the force or torque is maintained essentially constant: With decreasing loading of the consumer and thus a dropping load pressure the opening width of the control valve is enlarged due to the constant control pressure to a degree that the load pressure is again brought to the original level due to the volume stream thus increased and the originally set pressure difference is again achieved. However, the movement speed of the consumer is thus increased. An increase in the movement speed can also occur, for example, also if the consumer described is connected to a pump adjustable in stroke volume, which handles one or more additional consumers. Now if the first consumer is operated under a definite load pressure with a fully open control valve, i.e., with the maximum intended movement speed, where the pump pressure is greater by a given amount than the load pressure and if it further increases due to a corresponding loading of a second consumer, the movement speed of the first consumer would exceed the maximum intended value. One shortcoming of the drive system described consists in the fact that apart from speed losses due to friction, the movement speed of the consumer is not limited. This can result in excessive r.p.m.'s in a consumer designed as a hydraulic motor and to destruction of the hydraulic motor.